The present invention relates to photoelectric measuring systems of the type comprising a graduation carrier which defines a measuring graduation, a scanning plate mounted to move along the graduation, an illuminating arrangement mounted to illuminate the scanning plate, at least one photosensor mounted to detect light from the illuminating arrangement modulated by the graduation and the scanning plate, and an evaluating unit coupled to the photosensor.
Such systems are often used as incremental length or angle measuring systems, and they often include devices for photoelectrically generating electrical signals that can be used for example to define a zero point for the measuring system in a reproducible manner.
West German DE-OS No. 18 14 785 describes the construction of a reference mark for the generation of a reference pulse.
Using this approach however a sufficiently exact reference pulse can be derived from such a reference mark only if the scanning distance is maintained at a very small magnitude. For this reason, using this approach the scanning distance must be held to close tolerances with respect to undesired distance fluctuations.
In the prior art photoelectric scanning of known reference marks there are typically obtained one-beat signals (Eintaktsignale). In order to obtain the push-pull or psuedo-push-pull signals required for sure evaluation, two reference marks or a reference mark and a field (such as for example a mirror) must be provided and scanned to generate a reference signal. In the event two reference marks or a reference mark and a field are used, uneven dirtying of the reference marks or relative spacing differentials between the reference marks in the scanning may cause the generated photoelectric signal to change in such a way that a sure evaluation of the reference mark is no longer possible.
The theory and the interrelations between the various components of such path measuring systems are thoroughly explained in the dissertation published in 1978 by J. Willhelm entitled "Dreigitterschritgeberphotoelektrische Aufnehmer zur Messung von Lageanderungen" (TU Hannover).
In addition, West German DE-OS No. 23 16 248 describes a photoelectric pulse generator that operates with phase grids, which allow a greater scanning distance between the two grids which are shiftable with respect to one another. For this reason, the sensitivity to spacing changes is less critical than in the reference marks described initially above. However, this document provides no suggestion as to how a reference mark can be scanned and evaluated with sufficient certainty.